This invention relates to improvements in and relating to reporting cell reception measurement information from a user terminal in a cellular satellite telecommunications system, and has particular application to a user terminal for the telecommunications system and also a remote subsystem for communicating with the user terminal.
Terrestrial mobile telecommunications systems are well known and a number of different systems have developed which operate according to different standards. These public land mobile networks (PLMNs) may operate according to analog or digital standards. In Europe, the Far East, excluding Japan and elsewhere, the Global System Mobile (GSM) network has become popular, whereas in USA, the Advanced Mobile Phone Service (AMPS) and the Digital American Mobile Phone System (DAMPS) are in use, and in Japan, the Personal Handiphone System (PHS) and the Personal Digital Communication (PDC) network are used. More recently, proposals have been made for a Universal Mobile Telecommunications System (UMTS). These networks are all cellular and land-based with transmitter/receivers which communicate with mobile user terminals.
Considering for example the GSM system, individual cells of the mobile network are served by a series of geographically spaced, terrestrial base station subsystems (BSS) which each comprise a base transceiver stations (BTS) which are coupled through base station controllers (BSCs) to a mobile switching centre (MSC) which may provide a gateway out of the network to a conventional public switched telephone network (PSTN). The PLMN includes a home location register (HLR) which stores information about the subscribers to the system and their user terminals (UTs). When a UT is switched on, it registers with the HLR. If the user roams to a different GSM network, the user terminal registers with a visitor location register (VLR) of the visited network, which communicates with the HLR of the home network for routing and other purposes. DAMPS, PHS and PDC networks have a generally similar architecture.
The user terminal is operable in an idle mode when no call is being made, and a so-called dedicated mode for the duration of a call. The telephone call is routed to the subscriber""s UT through the MSC, to the BSC and then to the selected BTS, which provides a full duplex channel to the UT. The channel comprises a downlink channel from the BTS to the UT and an uplink channel from the UT to the BTS. The channels include time division multiple access (TDMA) time slots on frequencies in a hopping sequence allocated on initiation of the call.
In a GSM system, the telephone call is directed over a dedicated traffic channel or TCH. Each TCH has an associated slow rate control channel or SACCH, which is interleaved with the TCH. These channels are described in more detail on pp 195-201 of xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by M. Mouly and M-B. Pautet, 1992 Cell and Sys, ISBN: 2-9507190-0-7.
Also, a number of common channels are provided for all UTs within a particular cell. A broadcast control channel BCCH is broadcast from the BTS to all UTs within a particular cell, which provides information that identifies the cell to each UT. The BCCH is received by the UT in the idle mode i.e. when no call is being made. Each cell has is own BCCH, so the relative signal strengths of the BCCHs at the UT can be used to determine which cell can best be used for TCH/SACCH communication with the UT. Other system information may be transmitted to the UTs of a particular cell in the BCCH.
The SACCH communicates control information between the UT and the BSS. In particular, the SACCH is used for power control, so as to control the power of signals transmitted by the BTS to the UT, and to control the power of signals transmitted from the UT to the BTS, in order to conserve battery power in the UT.
On commencement of a call, initial UT power level settings are transmitted from the MSC/BSC in the downlink SACCH to the BTS and the UT. These initial settings are default values to get the power control process up and running at the start of the call. The UT then periodically measures the quality of the received signals on the downlink, for example, every 0.5 sec. and the measured values are transmitted back on the SACCH uplink to the BTS and then to the BSC. The BSC includes processing circuitry to update the power settings and appropriate instructions are then communicated from the BSC to the BTS and the UT using the downlink SACCH in order to update their power settings.
The quality measurements carried out at the UT consist of successive measurements of the bit error rate (BER) in signals received from the BTS through the downlink, the measurement values being an average of the BER over a measurement period e.g. 0.5 sec.
The BSC uses these measurement values and its own measurements of received UT uplink emissions to calculate a desired power transmission level for the BTS and the UT, the desired power level being computed as a function of the average BER.
A number of different mobile telecommunication systems have been proposed that use satellite communication links to the mobile user terminals. One network known as the IRIDIUM(trademark) satellite cellular system is described in for example EP-A-0365885 and U.S. Pat. No. 5,394,561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets are configured to establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.
Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed with an orbital radius in the range of 10-20,000 km and reference is directed to Walker J. G. xe2x80x9cSatellite Patterns for Continuous Multiple Whole Earth Coveragexe2x80x9d Royal Aircraft Establishment, pp 119-122 (1977). These orbits are also known as intermediate earth orbits (IEOs). The ICO(trademark) satellite cellular system described for example in GB-A-2 295 296 includes a constellation of MEO satellites. Communication does not occur between adjacent satellites and instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to a land-based telephone network. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM.
In satellite communications networks, ground stations are located at different sites around the world in order to communicate with the orbiting satellites. In the ICO(trademark) system and others, a visitor location register is associated with each of the satellite ground stations, which maintains a record of the individual user terminals that are making use of the particular ground station.
It would be advantageous in a satellite telecommunication system, to control the power level of the individual UTs and the power of the satellite transmission which conveys signals from the satellite to the UT. This would conserve battery power for the UT and also would conserve power in the satellite, which has a limited power supply from its solar panels and batteries. It has therefore been proposed to include a power control arrangement, which makes use of measured values of a quality metric for signals received at the UT so that the satellite and UT can be commanded to operate at an optimum power level. However, at the initiation of a call, the use of initial default power setting levels as in a conventional terrestrial GSM system previously described, would give rise to certain problems. With satellite systems, the transit time for signals between the ground station, the satellite and the UT is significantly greater than the corresponding transit times for terrestrial mobile networks, so that if the prior GSM protocol were adopted, there would be a significant delay at the start of a call before a measured value of received signal quality was used to set the transmission power, because the system would first have to use initial default power setting values and wait for measured values to be reported from the UT. This would involve sending the description of neighbouring cells to be measured, to the UT, followed by a measuring procedure for the cells, followed by reporting the outcome from the UT to the ground station, which would be wasteful of satellite and UT power. Also, the range of power level variation needed is greater than with terrestrial systems and so the default initial power setting values would need to be greater than with a terrestrial system in order to ensure satisfactory performance at the start of a call, which would also be wasteful of power.
Also, there would be a significant delay at the commencement of a call before data would be collected from the UT at the ground station to determine the quality of reception at the UT, which gives rise to difficulties. For example, in a satellite system, the user terminal may be able to communicate via cells provided by different satellites concurrently, and it may be desirable to route the call through one of them to avoid call traffic congestion in the other satellite, but this decision cannot be taken without data concerning the reception quality and hence cannot readily be taken at the start of the call.
The present invention involves reporting cell reception measurement information from a user terminal in a cellular telecommunications system, in which a cell reception measurement at the user terminal is made in an idle mode when no call is being made. Prior to commencement of a call, measurement information relating to the measurement made in the idle mode is reported to a location remote from the user terminal, and characteristics of signals transmitted to the user terminal for the call are set in dependence upon the report.
Thus, the delays that would be associated with prior art systems are reduced at the start of the call.
The present invention thus provides a method of reporting cell reception measurement information from a user terminal in a cellular telecommunications system, comprising making a cell reception measurement at the user terminal in an idle mode when no call is being made, and prior to commencement of a call, reporting measurement information relating to the measurement made in the idle mode to a location remote from the user terminal, for setting characteristics of signals transmitted to the user terminal for the call.
The report may be made when the user terminal switches into a dedicated mode to make a call and may comprise a report of a measurement of the quality of reception of a cell broadcast channel in the idle mode.
A measurement of cell broadcast channels for a number of different cells may be to provide the measurement information, to permit a decision to be made concerning a cell handover for the call.
A determination may be made of the cell with the best reception quality, and data concerning the best cell and its reception quality is reported in the measurement information.
The method according to the invention can be used for a satellite system wherein cells are made available to the user terminal by more than one satellite concurrently, and the method may include determining the best cell for each available satellite and providing data concerning the best cells and the reception quality thereof in the reported measurement information. Thus, data is provided which allows a choice to be made concerning the satellite to be used for the call, for example to relieve traffic congestion for one of the satellites.
The invention also provides a user terminal for use in a cellular telecommunications system, comprising receiver circuitry configured to make a cell reception measurement in an idle mode when no call is being made, and transmitter circuitry operable prior to commencement of a call, to report measurement information relating to the measurement made in the idle mode to a location remote from the user terminal, for setting characteristics of signals transmitted to the user terminal for the call.
In another aspect, the invention provides a method of operating a subsystem for use in a cellular telecommunications system to communicate with remote user terminals, comprising receiving from a remote user terminal, a report of cell measurement information corresponding to a cell reception measurement made by the user terminal in an idle mode when no call is being made, the report being made to the subsystem prior to commencement of a call, and setting characteristics of signals transmitted to the user terminal for the call in dependence on the report.
The power level for signals for the call may be set in dependence upon the report. Also, a decision to make a cell handover and a choice of cell for the handover may be made on the basis of the report.
The invention also includes a subsystem for use in a cellular telecommunications system to communicate with remote user terminals, comprising: means to receive from a remote user terminal, a report of cell measurement information corresponding to a cell reception measurement made by the user terminal in an idle mode when no call is being made, the report being made to the subsystem prior to commencement of a call, and setting means to set characteristics of signals transmitted to the user terminal for the call in dependence on the report.
The subsystem may comprise a ground station for a satellite telecommunications system, wherein the setting means is operable to transmit a setting signal towards a satellite for controlling a transmission characteristic thereof for the call. The cell report information may include data concerning diverse communication paths for the call to the user terminal via different satellites, and the setting means may be operable to select one of the paths for the call.
In another aspect, the invention provides a signal transmitted by a user terminal for a cellular telecommunications system, at the start of a call, comprising a speech channel with an associated control channel, a message concerning the quality of reception at the user terminal measured prior to the call being transmitted in the control channel. The message may include data concerning the quality of reception for different cells of the system.